BLYTHEVILLE, ARK. — Clay Mayes slams on the brakes of his Chevy Silverado and jumps out with the engine running, yelling at a dogwood by the side of the dirt road as if it had said something insulting.

Its leaves curl downward and in on themselves like tiny, broken umbrellas. It’s the telltale mark of inadvertent exposure to a controversial herbicide called dicamba.

The damage here in northeast Arkansas and across the Midwest — sickly soybeans, trees and other crops — has become emblematic of adeepening crisis in American agriculture.

Farmers are locked in an arms race between ever-stronger weeds and ever-stronger weed killers.

The dicamba system, approved for use for the first time this spring, was supposed to break the cycle and guarantee weed control in soybeans and cotton. The herbicide — used in combination with a genetically modified dicamba-resistant soybean — promises better control of unwanted plants such as pigweed, which has become resistant to common weed killers.

The problem, farmers and weed scientists say, is that dicamba has drifted from the fields where it was sprayed, damaging millions of acres of unprotected soybeans and other crops in what some are calling a man-made disaster. Critics say that the herbicide was approved by federal officials without enough data, particularly on the critical question of whether it could drift off target.

Government officials and manufacturers Monsanto and BASF deny the charge, saying the system worked as Congress designed it.

Leaves and a stalk from a soybean plant showing signs of being affected by dicamba. (Andrea Morales/For The Washington Post)

The backlash against dicamba has spurred lawsuits, state and federal investigations, and one argument that ended in a farmer’s shooting death and related murder charges.

“This should be a wake-up call,” said David Mortensen, a weed scientist at Pennsylvania State University.

Herbicide-resistant weeds are thought to cost U.S. agriculture millions of dollars per year in lost crops.

After the Environmental Protection Agency approved the updated formulation of the herbicide for use this spring and summer, farmers across the country planted more than 20 million acres of dicamba-resistant soybeans, according to Monsanto.